FiOS was installed last Friday. Had been a Verizon residential telephone only customer for a long time, so had pre-existing Verizon ID before FiOS was installed. Subscribed to HBO when purchased the FiOS TV package.
From HBOGO.com/acitvate, chose ROKU then Verizon.
Next screen always says I need to subscribe to FiOS TV before activating HBO GO. Doesn't seem to be any way out of there. I already am a FiOS TV + HBO subscriber.